1. Field of the Invention
The present invention relates to a method for forming a hole such as a through hole on a printed circuit board with a laser, and a hole forming laser apparatus.
2. Related Background Art
Formation of holes for mounting electronic parts, such as through holes, on a printed wiring board has generally been conducted with a mechanical method utilizing a drill. In recent years, however, methods for hole formation with a laser beam have been developed in order to respond to the requirements for forming smaller holes resulting from the increasing density of the printed circuit pattern, and non-through holes. Such technology is already known, for example, in the Japanese Patent Laid-Open Applications Nos. 61-95792 and 62-216297. In order to further reduce the working time in such known technology, there is also proposed a method of splitting a laser beam to plural working optical axes with plural partially transmitting mirrors of different reflectances. Improvements in efficiency and cost are necessary to realize commercial application of the hole forming technology with a laser beam also and the above-mentioned method of laser beam splitting to plural optical axes is effective for this purpose.
However, in the latter method mentioned above, the splitting of a laser beam into plural optical axes results in a lowering of the output (intensity) of the laser beam. For example, if a laser beam is split equally into four beams, the intensity of each beam becomes a quarter of the original intensity. Since the working speed is faster and the quality of formed hole becomes higher as the energy per unit time is higher in the laser beam, the above-mentioned conventional method is unable to provide the expected improvement in the working efficiency because of the slower working speed for each hole. Besides, the quality of the formed hole deteriorates due to the lowered beam intensity.